I think the answer is 3, when 3 of the points describe a triangle and the other 2 points are inside the triangle. The 3 points of the triangle form a 1-set. The 2 interior points cannot be members of a 1-set.

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Given a collection of points P in the plane, a 1-set is a point in P that can be separated from the rest by a?

Given a collection of points P in the plane, a 1-set is a point in P that can be separated from the rest by a line, .i.e the point lies on one side of the line while the others lie on the other side.
The number of 1-sets of P is denoted by n1(P). The minimum value of n1(P) over all configurations P of 5 points in the plane in general position (.i.e no three points in P lie on a line) is